How to unpack 4 bytes of binary data as 3 byte and 1 byte values?
I have 4 bytes of binary data (big-endian) that I want to unpack. If it contained two 2-byte unsigned integer values, this would be straightforward:
a, b = data.unpack("C>C>")
But what if the data contains a 3-byte value (a) followed by a 1-byte value (b)? The unpack method doesn't seem to be able to handle formats other than 8-, 16-, 32-, and 64-bit integers. This is what I came up with:
a, b = data.unpack("L>XC") # "L>": unpack a 32-bit unsigned int (big-endian) # "X": rewind (skip back) one byte # "C": unpack an 8-bit unsigned int a >>= 8 # drop the last (lowest) byte from a
(If the data were little-endian, a &= 0xFFFFFF could be used to drop the last (highest) byte instead.)
Is there a more elegant way to unpack these values?
That's a reasonable way. Another way (that doesn't involve backing up) would be
a, b, c = data.unpack("S>CC") # C doesn't have endianness ab = a << 8 + b
Since your values are unsigned, you don't need to worry about sign extension when sticking them together.
And for completeness, you could also go in the opposite direction — unpack a single 32-bit int and split it up using bit operations.
ab, = data.unpack("L>") a, b = ab >> 8, ab & 0xFF
@hobbs has a good answer. I just wanted to mention that you can also use Numeric#divmod in this case:
ab, = data.unpack('L>') a, b = ab.divmod(2**8)
a, b = data.unpack('L>').divmod(2**8)